The present invention relates to electrochemical sensors for the concentrations of species in a fluid mixture and in particular to a system for regulating the richness of an air-fuel gaseous mixture.
One well known type of electrochemical sensor operates on the principle of a concentration cell and the measurement of the partial pressure of one or more species of the gaseous mixture to be analysed. The gaseous mixture which is present in a first compartment, for example an oxygen-inert gas mixture, is separated from a reference medium by the wall of a solid electrolyte, whereof each face carries an electrode. As is known, the equations governing these sensors are:
at the electrode/electrolyte interfaces: ##STR1## The voltage V.sub.E1/E2 which then develops between the electrodes is given by the NERNST law: ##EQU1## with R=constant of perfect gases=8.314 (mole. .degree.K.)
F=Faraday number=96,490 C PA1 T=absolute temperature in degrees Kelvin
P.sub.1 and P.sub.2 =partial pressures of 1 and 2 in compartments 1 and 2.
In the case where the mixture is reactive, e.g. a mixture of O.sub.2 and CO and if the electrode is a catalyst of the reaction of these gases, the following reaction occurs: EQU 2CO+O.sub.2 .revreaction.2CO.sub.2 ( 3)
and finally if combustion is complete up to the time of achieving the reversible thermodynamic equilibrium the following relation is proved: ##STR2## with K(T) a temperature-dependent equilibrium coefficient and CO.vertline.O.sub.2 .vertline..sup.1/2, CO.sub.2 the partial pressures of carbon monoxide, oxygen and carbon dioxide.
Recent approaches in connection with the construction of sensors propose the use of a means combining the electrode function and the reference medium function. For this purpose, an electrode base on a combination of the following type is used: M-MX in which M is a metal and X oxygen or a halogen to be detected (e.g. M-MO in the case of the detection of oxygen). As an improvement to this approach, sensors have also been proposed which are based on the method of thin layers used in microelectronics. These two construction types, among others, make it possible to in part overcome the parasitic effects of temperature on the response curve of the sensors. Thus, on the basis of equations (3) and (4) the value of V.sub.E1/E2 is doubly dependent on the "temperature" parameter. Thus, an appropriate choice on the basis of the formation heat tables of M-MX makes it possible to partly compensate the two temperature-sensitive terms.
The invention relates to sensors of the type comprising electrodes or other means placed upstream of the electrochemical measuring cell carrying out complete catalysis, in such a way that the gaseous mixture to be analysed reaches thermodynamic equilibrium at least at the level of the electrode-electrolyte interface and whereof one of the electrodes is of the type described hereinbefore on the basis of a combination M-MX.
Such sensors are used in regulating internal combustion engines and particularly for regulating the admission of the air-fuel mixture to the carburettor or the fuel injector. The sensor is then placed in the exhaust circuit and analyses the relative concentration of the oxygen and carbon monoxide contained in the gas. The sensor must then be adapted to certain characteristics specific to this use. Thus, the exhaust gas arrives in jerks with the timing of the alternating movement of the different pistons. These problems are solved by taking samples of the gas to be analysed and only these are introduced into the sensor where they are brought into thermodynamic equilibrium. The term "assay" is used and if they are sufficiently close together there is a tendency towards a continuous analysis, although in reality the sensor functions under dynamic conditions. To arrive at this result, means are generally positioned upstream of the sensor which selectively decelerate the gas passing through them and which limit the gaseous exchange between the external medium and the interior of the sensor. By analogy with the laws governing electric circuits these means can be called "transfer impedance means". Various solutions have been proposed and they are based on two approaches. According to the first approach, the gaseous mixture to be analysed enters the sensor by one or more metering holes. According to the second approach, the gaseous mixture traverses a porous solid material, this being generally the case with sensors having a planar structure produced by the thin layer principle. The porous material may also coincide with the extension of the measuring electrode. According to a particularly advantageous variant of this prior art sensor, the electrode is in the form of a layer of porous catalyst material and the gas to be analysed propagates within the same in a direction parallel to the largest dimensions of the electrode before reaching the actual measuring area. Such a sensor is described in U.S. Pat. No. 4,271,000.
The sensors, whose operation and characteristic principles have been described hereinbefore, have response curves which exhibit a sudden dip when the stoichiometry of the gaseous mixture to be analysed is reached. Moreover, the response of these sensors consists of a set of curves and not a single curve and said set is parametrized in temperature due to the aforementioned dependence and essentially coincide in the dip region. Therefore, it is usually possible to use this part of the response curves and consequently it is only possible to reliably and repetitively detect the stoichiometry of the reaction defined by relation (3).
These sensors can be used as they are in certain countries, particularly countries imposing severe anti-pollution regulations. The motor then operates with a stoichiometric air-fuel mixture.
Other countries, such as certain of the European countries, impose a lean mixture, particularly as a result of fuel economy measures. Thus, the prior art sensors with an "all or nothing" response curve for a single regulating point cannot be used for this type of operation. To solve this problem and more generally modify the regulation point two approaches have been proposed.
According to the first approach, it is possible to partly linearize the response curve by modifying certain components of the sensors, particularly by adopting a special measuring electrode construction. Such sensors are described in European Patent Application EP-A-0018871, published on November 12, 1980. The device described therein is sensitive to carbon monoxide and not to oxygen. This is advantageous because the carbon monoxide content variation dynamics are generally greater than that for oxygen in the exhaust gases. However, this device can only be used in a restricted range of relative concentrations directly beyond which parasitic effects of temperature become too great for ensuring an adequate measuring precision.
According to the second approach, "the transfer impedance" as defined hereinbefore is made selective by the use of a porous material especially adapted to the fluid to be analysed.
Thus, in the case of a gaseous mixture of two species, for example oxygen and carbon monoxide, one of the species can diffuse more rapidly than the other within the porous material in question. In other words, to a given mixture composition on entering the sensor, may correspond a different mixture composition within the porous material and then at the measuring electrode.
By integrating into the sensor a predetermined selective transfer impedance associated with catalysis means for bringing the gaseous mixture into thermodynamic equilibrium prior to analysis, the apparent concentration of the species of a gaseous mixture is measured, i.e. that at the measuring electrode and consequently the regulation point is displaced on either side of the stoichiometry of the gaseous mixture which in actual fact circulates in the exhaust pipes.
A regulating system using this type of sensor described in European Patent Application No. 0 0115 30, published on May 28, 1980, is less dependent on the temperature effects because in this case detection also takes place of the dip in the response curve and which occurs on either side of the stoichiometry as a function of the nature of the "transfer impedance", i.e. in a region of the curve which is less sensitive to this parameter. However, the displacement amplitude permitted by these devices is relatively limited and is fixed once and for all at a predetermined value at the time of manufacture.
In certain applications, particularly in systems for regulating the richness of the air-fuel mixture, entering the cylinders of an internal combustion engine controlled by a digital or analog computer, it is necessary to be able to develop the regulation point as a function of parameters measured by different sensors, such as the temperature or speed of the vehicle driven by the engine and this takes place in a continuous manner in accordance with predetermined laws or laws produced by the computer.